This invention relates generally to thin film semiconductor materials. Within the context of this disclosure, thin film semiconductor materials comprise materials which are deposited by building up thin layers on a substrate, typically through a vapor deposition process. Such processes include plasma deposition processes (also referred to as plasma chemical vapor deposition processes), wherein a process gas, typically comprised of a semiconductor precursor and generally a diluent gas, is subjected to an electrical field which ionizes the gas so as to create a reactive plasma. The plasma decomposes at least some components of the process gas and deposits a layer of semiconductor material onto a substrate maintained in, or in close proximity to, the plasma. Nonplasma vapor deposition processes such as nonplasma chemical vapor deposition and evaporation processes may be similarly employed for the preparation of thin film semiconductor materials.
Thin film semiconductor materials are generally considered to be disordered semiconductor materials insofar as they are not single crystalline materials. Within the context of this disclosure, the terms "thin film semiconductor materials" and "disordered semiconductor materials" shall be used interchangeably to refer to those semiconductor materials which are lacking in long range order (i.e. are not in a single crystalline state). Thin film semiconductor materials can have short range order or intermediate range order, and may comprise amorphous semiconductor materials as well as nanocrystalline and microcrystalline semiconductor materials.
The degree of ordering has been found to be an important characteristic of thin film semiconductor materials. For example, in the field of photovoltaic devices, it has been found that the best devices are manufactured when photogenerative material (for example the intrinsic layer in a P-I-N device) is amorphous, but is prepared from material obtained under deposition conditions just below the threshold of microcrystalline growth. See, for example, Tsu et al., Applied Physics Letters Vol. 71, No. 10, 1317-1319 (Sep. 8, 1997). Deposition at such conditions near amorphous microcrystalline threshold is generally desirable. Within the context of this disclosure, operation near the amorphous/crystalline threshold means that deposition parameters including process gas composition, process gas pressure, power density, substrate temperature and the like are selected so as to have values which approach those which produce microcrystalline material, but are such that the material is not microcrystalline.
Amorphous material produced under these conditions has good medium range order, and photovoltaic devices produced therefrom have good operational characteristics such as open circuit voltage, fill factor and the like. In addition, these materials have good stability, and are resistant to the formation of light induced defects. As the material becomes microcrystalline, open circuit voltage of the photovoltaic devices decreases. In addition, the presence of grain boundaries in microcrystalline material can adversely affect the performance of the photovoltaic devices. Likewise, if the semiconductor material is deposited so as to be more disordered, problems of device efficiency and stability also arise. Therefore, the photogenerative layers of thin film photovoltaic devices are most preferably fabricated under deposition conditions which are near the amorphous microcrystalline threshold.
The art has not heretofore recognized that this amorphous microcrystalline threshold will vary as the semiconductor layer is being deposited even if all deposition parameters are maintained constant. Therefore, materials prepared according to prior art processes have been less than optimum because of this shift in the amorphous/microcrystalline threshold. Furthermore, photogenerative materials are often fabricated with a graded composition, for example to give profiled band gap, and it has also been found that such compositional gradation can also affect the amorphous microcrystalline threshold. Therefore, the present invention recognizes the fact that the amorphous/microcrystalline threshold for a particular deposition process will shift as a result of semiconductor layer thickness as well as composition variations; and accordingly, the present invention adjusts deposition parameters so as to maintain deposition conditions at the ideal near-threshold level for the entirety of the deposition process. As will be demonstrated hereinbelow, material produced by the present invention is optimized, as is demonstrated by the operational parameters of photovoltaic devices produced therefrom.